Generally speaking, particle-optical apparatus, such as electron microscopes or electron-lithography apparatus, are arranged to irradiate an object to be studied or worked by means of a beam of electrically charged particles (usually an electron beam) which is produced by means of a particle source such as a thermionic electron source or an electron source of the field-emission type. Irradiation of the object may be aimed at imaging it for the purpose of examination in such apparatus (specimens in electron microscopes) or may be aimed at forming very small structures on the object, for example for microelectronics (electron lithography apparatus). In both cases focusing lenses are required for focusing the electron beam.
The electron beam can in principle be focused by means of two methods. According to the first method the area to be imaged of a specimen to be examined is more or less uniformly irradiated by the electron beam and an enlarged image of the specimen is formed by means of the focusing lens unit. The focusing lens unit is in that case the objective lens of an imaging lens system; the resolution of the objective lens then determines the resolution of the apparatus. Apparatus of this kind are known as Transmission Electron Microscopes (TEM). According to a second focusing method, the emissive surface of the electron source, or a part thereof, is imaged, generally strongly reduced, on the specimen to be examined (in the Scanning Electron Microscope or SEM) or on an object on which the microstructure is to be formed (in the lithography apparatus). The image of the electron source (the "spot" which is moved across the surface of the object in a predetermined scanning pattern by means of, for example deflection coils) is again formed by means of an imaging lens system. In the latter case the focusing lens unit is the objective lens of the spot-forming lens system; the resolution of this objective lens then decides the spot size of the beam and hence the resolution of the apparatus.
The lenses used in all apparatus of this kind are usually magnetic lenses, but may also be electrostatic lenses. Both types of lens are practically always rotationally symmetrical lenses. The behavior of such lenses inevitably is not ideal, i.e. they exhibit lens aberrations, among which the so-called spherical aberration and the chromatic aberration are usually decisive in respect of resolution of the lens; these lens aberrations thus determine the limit of the resolution of the known electron-optical apparatus. According to a fundamental theorem of particle-optics, such lens aberrations cannot be eliminated by compensation utilizing rotationally symmetrical electrical or magnetic fields.
In contemporary electron-optical apparatus, notably in scanning particle-optical apparatus comprising a spot-forming objective lens (the so-called Scanning Electron Microscope or SEM), there is a tendency to select the acceleration voltage of the electron beam so as to have a value which is lower than was customary thus far, i.e. of the order of magnitude of from 0.5 kV to 5 kV instead of the previously customary voltage of the order of magnitude of 30 kV or more. The reason for doing so is that at such comparatively low acceleration voltages the charging of non-conductive specimens (such as photoresist material in the case of manufacture of electronic integrated circuits) is substantially reduced; moreover, at these low voltages the so-called topographic contrast is substantially enhanced. At such low acceleration voltages the chromatic aberration is the major lens aberration, so the decisive factor in respect of resolution of the particle-optical apparatus. (This can be readily understood by considering the fact that the chromatic aberration is proportional to .DELTA.U/U, in which AU is the non-variable energy spread in the electron beam and U is the nominal acceleration voltage; this factor, therefore, increases as U is decreased.)
In order to enhance the resolution of the particle-optical apparatus nevertheless, it is already known to reduce said lens aberrations by means of a correction device having a non-rotationally symmetrical structure. Such a structure is known, for example from European Patent No. 0 373 399. The structure described therein is formed by a so-called Wien-type corrector, i.e. a structure in which a uniform electric field and a uniform magnetic field, extending perpendicularly thereto, are both oriented perpendicularly to the optical axis of the apparatus. This corrector is provided with a number of multipoles for the correction of the spherical aberration as well as the chromatic aberration, i.e. an electrical and a magnetic quadrupole, an electrical and a magnetic hexapole, and an electrical and/or a magnetic octupole.
An embodiment of the correction device according to the cited European Patent (described notably with reference to FIG. 5 and denoted therein by the reference numeral 20) enables correction of the chromatic aberration. This embodiment consists of a multipole unit which is formed by a number of electrical and magnetic poles whose pole faces are axially oriented, i.e. extend parallel to the optical axis of the apparatus. Each of said poles can be separately excited; by suitably choosing the individual excitations, therefore, a multipole unit thus constructed can form, as desired, a uniform electrical field extending perpendicularly to the optical axis and a uniform magnetic field which extends perpendicularly thereto, both fields extending perpendicularly to the optical axis; electrical and magnetic quadrupole fields, hexapole fields and an electrical and/or a magnetic octupole field can be superposed thereon.
In such a comparatively complex correction device it is extremely difficult to find the correct electrical and magnetic adjustment for the (very accurate) generation of said multipole fields. This difficulty becomes more serious as the number of multipole fields to be generated is greater, because each of said fields must have and retain exactly the adjusted correct value.
The cited European Patent No. 0 281 743 discloses a particle-optical apparatus which is provided with a focusing lens unit which is formed by a combination of at least two particle lenses and serves to focus a beam of electrically charged particles, and with a correction device for correcting chromatic and/or spherical aberration of the focusing lens unit. The correction unit described therein consists of at least four successively arranged octupole elements or twelve-pole elements. Therefore, in this correction device it will also be extremely difficult to find the correct electric and magnetic setting for generating said multipole fields.